We have investigated the cellular and antigenic requirements for incubation of secondary proliferative responses by human T lymphocytes. Two distinct properties of antigen-presenting peripheral blood mononuclear cells were studied: (a) the ability for appropriate cell surface constituents to construct an immunogenic moiety, and (b) the ability to present similar antigenic determinants when they are not covalently bound. Only Ia+ hapten-modified cells were effective stimulators. In contrast, both Ia+ and Ia- cell sonicates could stimulate secondary proliferative responses, but only in the presence of an accessory cell. This accessory cell was present in Ia+ macrophage, but not in Ia+ non-T lymphocyte, preparations. In contrast, macrophages or soluble factors produced by macrophages were not required for primed T cells to undergo hapten-specific proliferation in response to hapten-modified Ia+ stimulator cells. Thus, although all Ia+ cells tested can stimulate primed cells to proliferate, not all Ia+ cells can function as accessory cells for responses to sonicates. This may reflect the unique ability of a subpopulation(s) of Ia+ cells to bind or process sonicates or soluble antigens for appropriate recognition by primed T cells.